This invention relates to a sodium turbine pump, and more particularly to a vertical type sodium turbine pump.
A circulating pump using sodium as an intermediate heat medium is used for a fast breeder type atomic power plant. In this pump, the molten sodium is maintained at a temperature as high as 450.degree. C., and sodium becomes radioactive due to absorption of neutrons, so that a turbine pump finds its application as a circulating pump for this purpose. In this respect, a heat or radioactivity shielding member is disposed between a body proper of the pump and a drive means thereof, and a relatively long shaft has to be used for the pump.
As has been described earlier, a fluid to be driven by the sodium turbine pump is used for lubrication for a bearing.
The coefficient of viscosity of molten sodium is as low as 1/125 of that of turbine oil.
With a hydrodynamic bearing of an ordinary type, a load capacity of a film bearing is proportional to the square of a shaft diameter and a coefficient of viscosity of the fluid, but in inverse proportion to the square of a bearing clearance.
Accordingly, for obtaining the same load capacity as that of turbine oil in a hydrodynamic bearing, the bearing clearance should be reduced to about 1/14.6 ##STR1## or the radius of the shaft should be increased to about 14.6 times (.sqroot.215).
The former leads to a difficulty in a manufacturing technique, while the latter leaves a problem in manufacturing cost. As a result, a static-pressure bearing utilizing a discharge pressure from the pump is adopted for a sodium turbine pump. A sodium piping is directly connected to the bottom of a casing of a prior art vertical type sodium turbine pump. On the other hand, a sodium outlet nozzle is directly connected to another sodium piping.
An impeller is secured to the lower end of a shaft in a sodium turbine pump, while a sodium-suction side of the impeller is connected by way of a suction tube to a sodium pipe. A suction tube surrounds a mouth ring in an impeller.
An impeller casing consists of a cylindrical portion encompassing the impeller ring which extends from the impeller upwards in a cylindrical form, and a pan-shaped portion extending radially from the cylindrical portion towards the casing and shaft, with the shaft extending through the center of the pan-shaped portion.
A seal ring made of a stainless steel is disposed between the impeller casing and the casing, thereby sealing sodium tending to leake therebetween.
A bearing secured to the shaft provides a given clearance to a cylindrical portion of the impeller casing. Two or more holes extend through the cylindrical portion so as to permit part of sodium discharged from the pump to be supplied from outside of the cylindrical portion to the sliding surface of a bearing.
A diffuser is secured to an outer periphery of a discharge port of the impeller, with the opposite sides thereof being secured to the impeller casing and the suction tube.
A clearance between the impeller ring and the impeller casing as well as a clearance between the mouth ring and the suction tube are set to relatively small values so as to seal the sodium as tightly as possible. In like manner, a clearance between the sodium discharge portion of a impeller and the diffuser is set to a relatively small value for the same reason.
Since sodium flows through the respective clearances contiguous to the aforesaid bearing surface and sealing surface on a Poiseuille flow pattern, so that the flow rate of sodium leaking through the clearance is increased or decreased in proportion to a supply pressure of sodium and to the cube of a clearance on a bearing surface, sealing surface or the like, but in inverse proportion to the width of a land (a peripheral contacting surface excluding pockets for the bearing) which is provided on the bearing surface.
The following drawbacks are experienced with the prior art sodium turbine pump:
(1) Lowered volumetric efficiency of a pump PA0 (2) The construction of a pump is complex and a manufacturing cost thereof is high PA0 (3) Assembly, set-up and adjustment of a pump are difficult PA0 (4) Remarkable vibrations take place in a pump and thus its maintenance is encountered with difficulty:
A clearance, through which sodium is circulated, is provided on the bearing surface at one position, and on the sealing surface in two positions, thus providing many clearances, so that a circulating flow rate of sodium becomes a loss, with the result that a volumetric efficiency which is a ratio of a suction flow rate of processed sodium to a delivery flow rate of sodium, is lowered.
Since sodium to be processed is maintained at a high temperature and a long shaft has to be used, there results unwanted elongation of the shaft due to the use of high temperature sodium, thereby causing warping and an eccentric rotation of the shaft, with the accompanying metal-to-metal contact of a sliding surface of the bearing surface. A need to prevent or minimize the aforesaid metal-to-metal contact renders assembly, set-up and adjustment of the pump difficult.
The inflow of molten sodium pressurized by an impeller to the bearing surface and sealing surface exerts a complex vibratory force on the impeller. This vibratory force further promotes the vibrations due to the warping and eccentric rotation of the shaft, which may be attributed to the use of high-temperature molten sodium.